Prior to the inventions described in the aforementioned related patent applications, the liquid spray application of coatings, such as paints, lacquers, enamels, and varnishes, was effected solely through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from painting and finishing operations. For this reason, there has been a great deal of emphasis placed on the development of new coatings technologies which diminish the emission of organic solvent vapors. A number of technologies have emerged as having met most but not all of the performance and application requirements, and at the same time meeting emission requirements and regulations. They are: (a) powder coatings, (b) water-borne dispersions, (c) waterborne solutions, (d) non-aqueous dispersions, and (e) high solids coatings. Each of these technologies has been employed in certain applications and each has found a niche in a particular industry. However, at the present time, none has provided the performance and application properties that were initially expected.
Powder coatings, for example, while providing ultra low emission of organic vapors, are characterized by poor gloss or good gloss with heavy orange peel, poor distinctness of image gloss (DOI), and poor film uniformity. Moreover, to obtain even these limited performance properties generally requires excessive film thickness and/or high curing temperatures. Pigmentation of powder coatings is often difficult, requiring at times milling and extrusion of the polymer-pigment composite mixture followed by cryogenic grinding. In addition, changing colors of the coating often requires its complete cleaning, because of dust contamination of the application equipment and finishing area.
Water-borne coatings are very difficult to apply under conditions of high relative humidity without serious coating defects. These defects result from the fact that under conditions of high humidity, water evaporates more slowly than the organic cosolvents of the coalescing aid, and as might be expected in the case of aqueous dispersions, the loss of the organic cosolvent/coalescing aid interferes with film formation. Poor gloss, poor uniformity, and pin holes unfortunately often result. Additionally, water-borne coatings are not as resistant to corrosive environments as are more conventional solvent borne coatings.
Coatings applied with organic solvents at high solids levels avoid many of the pitfalls of powder and water-borne coatings. However, in these systems the molecular weight of the polymer has been decreased and reactive functionality has been incorporated therein so that further polymerization and crosslinking can take place after the coating has been applied. It has been hoped that this type of coating will meet the ever-increasing regulatory requirements and yet meet the most exacting coatings performance demands. However, there is a limit as to the ability of this technology to meet the performance requirement of a commercial coating operation. Present high solids systems have difficulty in application to vertical surfaces without running and sagging of the coating. If they possess good reactivity, they often have poor shelf and pot life. However, if they have adequate shelf stability, they cure and/or crosslink slowly or require high temperature to effect an adequate coating on the substrate.
Clearly, what was needed was an environmentally safe, non-polluting diluent that can be used to thin very highly viscous polymer and coatings compositions to liquid spray application consistency. Such a diluent would allow utilization of the best aspects of organic solvent borne coatings applications and performance while reducing the environmental concerns to an acceptable level. Such a coating system could meet the requirements of shop-applied and field-applied liquid spray coatings as well as factory-applied finishes and still be in compliance with environmental regulations.
Such a needed diluent was indeed found and is discussed in the aforementioned related applications which teach, among other things, the utilization of supercritical fluids or subcritical compressed fluids, such as carbon dioxide or nitrous oxide, as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute these compositions to application viscosity required for liquid spray techniques.
As used herein, it will be understood that a "supercritical fluid" is a material which is at a temperature and pressure such that it is above, at, or slightly below its "critical point" As used herein, the "critical point" is the transition point at which the liquid and gaseous states of a substance merge into each other and represents the combination of the critical temperature and critical pressure for a given substance. The "critical temperature", as used herein, is defined as the temperature above which a gas cannot be liquefied by an increase in pressure. The "critical pressure", as used herein, is defined as that pressure which is just sufficient to cause the appearance of two phases at the critical temperature. As used herein, a "compressed fluid" is a fluid which may be in its gaseous state, its liquid state, or a combination thereof depending upon the particular temperature and pressure to which it is subjected upon admixture with the composition which is to have its viscosity reduced and the vapor pressure of the fluid at that particular temperature, but which is in its gaseous state at standard conditions of 0.degree. C. and one atmosphere (STP). The compressed fluid may comprise a supercritical or subcritical fluid.
Also as used herein, the phrases "coating composition", "coating material", and "coating formulation" are understood to mean conventional coating compositions, materials, and formulations that have no supercritical fluid or subcritical compressed fluid admixed therewith. Also as used herein, the phrases "liquid mixture", "spray mixture", "coating mixture", and "admixed coating composition" are meant to include an admixture of a coating material, coating composition, or coating formulation with at least one supercritical fluid or at least one subcritical compressed fluid.
It is understood, of course, that the terms "coating composition", "coating material", and "coating formulation" are not limited to coatings that are only used to protect and/or enhance the appearance of a substrate, such as paints, lacquers, enamels, and varnishes. Indeed, the coating composition may provide a coating that acts as an adhesive or that is a release agent; a lubricant; a cleaning agent; or the like. Such coating compositions may also include those that are typically utilized in the agricultural field in which fertilizers, weed killing agents, and the like, are dispensed. Such coating compositions may also include those that are used to coat agricultural products such as fruits and vegetables or to coat pharmaceutical or medicinal products such as pills and tablets. The specific nature of the coating composition is not critical to the present invention provided that it can be admixed with the subcritical compressed fluid and then be sprayed.
Aforementioned U.S. Pat. No. 4,923,720 discloses processes and apparatus for the liquid spray application of coatings to a substrate that minimize the use of environmentally undesirable organic diluents. One of the process embodiments of that patent includes:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising:
(a) at least one polymeric compound capable of forming a coating on a substrate;
(b) at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application; and
(2) spraying said liquid mixture onto a substrate to form a liquid coating thereon.
That patent is also directed to a liquid spray process in which at least one active organic solvent (c) is admixed with (a) and (b) above prior to the liquid spray application of the resulting mixture to a substrate. The preferred supercritical fluid disclosed is supercritical carbon dioxide. The process employs an apparatus in which the mixture of the components of the liquid spray mixture can be blended and sprayed onto an appropriate substrate. The apparatus includes:
(1) means for supplying at least one polymeric compound;
(2) means for supplying at least one active solvent;
(3) means for supplying supercritical carbon dioxide fluid;
(4) means for forming a liquid mixture of components supplied from (1)-(3); and
(5) means for spraying said liquid mixture onto a substrate.
The apparatus may also provide for (6) means for heating any of said components and/or said liquid mixture of components.
U.S. patent application Ser. No. 631,680, filed Dec. 21, 1990, discloses processes for reducing the viscosity of compositions containing one or more polymeric compounds so as to make them transportable by adding a subcritical compressed fluid, which fluid is a gas at standard conditions of 0.degree. C. and one atmosphere pressure. Among these is a process for the liquid spray application of coatings to a substrate that minimizes the use of environmentally undesirable organic diluents and simultaneously reduces the pressure and/or temperature needed to achieve such a viscosity reducing diluent effect. One of the embodiments of that patent application includes a process for the liquid spray application of coatings to a substrate, which comprises:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising:
(a) at least one or more liquid polymeric compounds capable of forming a coating on a substrate wherein the number average molecular weight of the one or more liquid polymeric compounds is less than about 5,000 and
(b) at least one subcritical compressed fluid in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application, wherein the subcritical compressed fluid is a gas at standard conditions of 0.degree. C. and one atmosphere (STP); and
(2) spraying said liquid mixture onto a substrate to form a liquid coating thereon having substantially the composition of the said coating formulation.
That patent application is also directed to a liquid spray application process in which at least one active solvent is added in which one or more of the polymeric compounds are at least partially soluble and which is at least partially miscible with the subcritical compressed fluid. In a preferred embodiment, the subcritical compressed fluid is carbon dioxide. Liquid polymeric compounds are not required in this alternative embodiment. In yet another embodiment, the coating compositions may also contain one or more polymeric compounds having higher number-average molecular weights provided that at least 75 weight percent of the total weight of all polymeric compounds has a weight-average molecular weight of less than about 20,000.
Smith, in U.S. Pat. No. 4,582,731, issued Apr. 15, 1986, U.S. Pat. No. 4,734,227, issued Mar. 29, 1988, and U.S. Pat. No. 4,734,451, issued Mar. 29, 1988, discloses methods and apparatus for the deposition of thin films and the formation of powder coatings through the molecular spray of solutes dissolved in supercritical fluid solvents, which may contain organic solvents. The concentration of said solutes are described as being quite dilute; on the order of 0.1 percent. In conventional coating applications, the solute concentration is normally 50 times or more greater than this level.
The molecular sprays disclosed in the Smith patents are defined as a spray "of individual molecules (atoms) or very small cluster of solute" which are in the order of about 30 Angstroms in diameter. These "droplets" are more than 10.sup.6 to 10.sup.9 less massive than the droplets formed in conventional application methods that Smith refers to as "liquid spray" applications.
Sievers, et al., in U.S. Pat. No. 4,970,093, issued Nov. 13, 1990, disclose a method for depositing a film of a desired material on a substrate, which comprises dissolving at least one reagent in a supercritical fluid comprising at least one solvent. Either the reagent is capable of reacting with or is a precursor of a compound capable of reacting with the solvent to form the desired product, or at least one additional reagent is included in the supercritical solution and is capable of reacting with or is a precursor of a compound capable of reacting with the first reagent or with a compound derived from the first reagent to form the desired material. The supercritical solution is rapidly expanded to produce a vapor or aerosol and a chemical reaction is induced in the vapor or aerosol so that a film of the desired material resulting from the chemical reaction is deposited on the substrate surface.
The process disclosed in the Sievers, et al. patent utilizes dilute solutions of reagent(s) in the form of a supercritical solution to apply thin solid films by reaction of the vapor or aerosol that results from rapid expansion of the supercritical solution. The patent teaches that selection of a suitable supercritical solvent is an important feature, because the supercritical solvent must expand rapidly with minimal droplet and molecular cluster formation, in order to allow formation of a homogeneous, nongranular film by reaction. After rapid expansion, the solution is substantially vaporized creating individual molecules or small clusters of the reagents and solvent, which can rapidly participate in chemical reactions at or near the substrate surface to form the desired film. That is, a molecular spray such as taught by Smith is desired and a liquid spray is avoided.
Coating compositions are commonly applied to a substrate by passing them under pressure through an orifice into air in order to form a liquid spray, which impacts the substrate and forms a liquid coating. In the coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray uses compressed air to break up the coating composition into droplets and to propel the droplets to the substrate. The most common type of air nozzle mixes the coating composition and high-velocity air outside of the nozzle to cause atomization. Auxiliary air streams modify the shape of the spray. The coating composition flows through the orifice in the spray nozzle at a low pressure, typically less than 18 psi. Air spray is used to apply high quality coatings because it produces fine droplet size and a "feathered" spray, that is, the spray has a uniform interior and tapered edges. This is particularly desirable so that adjacent layers of sprayed coating can be overlapped to form a coating with uniform thickness. However, because of the high air volume, air spray deposits the coating inefficiently onto the substrate, that is, it has low transfer efficiency, which wastes coating. Furthermore, air spray uses a large concentration of organic solvents to produce the low viscosity needed for atomization, which causes air pollution.
Airless spray uses a high pressure drop across the spray orifice to propel the coating composition through the orifice at high velocity. Upon exiting the orifice, the high-velocity liquid breaks up into droplets and disperses into the air to form a liquid spray. The momentum of the spray carries the droplets to the substrate. Spray pressures typically range from 700 to 5,000 psi. The spray tip is contoured to modify the shape of the liquid spray, which is usually a round or elliptical cone or a flat fan.
The conventional atomization mechanism of airless sprays is well known and is discussed and illustrated by Dombroski, N. and Johns, W. R., Chemical Engineering Science 18: 203, 1963. The coating exits the orifice as a liquid film that becomes unstable from shear induced by its high velocity relative to the surrounding air. Waves grow in the liquid film, become unstable, and break up into liquid filaments that likewise become unstable and break up into droplets. Atomization occurs because cohesion and surface tension forces, which hold the liquid together, are overcome by shear and fluid inertia forces, which break it apart. This process is shown photographically for an actual paint in the brochure entitled "Cross-Cut.TM." "Airless Spray Gun Nozzles", Nordson Corporation, Amherst, Ohio. As used herein, "liquid-film atomization" and "liquid-film spray" refer to a spray, spray fan, or spray pattern in which atomization occurs by this conventional mechanism.
In liquid-film atomization, however, the cohesion and surface tension forces are not entirely overcome and they profoundly affect the spray, particularly for viscous coating compositions. Conventional airless spray techniques are known to produce coarse droplets and defective spray fans that limit their usefulness to applying low-quality coating films. Higher viscosity increases the viscous losses that occur within the spray orifice, which lessens the energy available for atomization, and it decreases shear intensity, which hinders the development of natural instabilities in the expanding liquid film. This delays atomization so that large droplets are formed The spray also characteristically forms a "tailing" or "fishtail" spray pattern. The surface tension and cohesive forces in the liquid film tend to gather more liquid at the edges of the spray fan than in the center, which produces coarsely atomized jets of coating. Sometimes the jets separate from the spray and deposit separate bands of coating. At other times, they thicken the edges so that more coating is deposited at the top and bottom than in the center of the spray. These deficiencies produce a non-uniform deposition pattern that makes it difficult to apply a uniform coating. The fishtail sprays are generally angular in shape and have a relatively narrow fan width, that is, a fan width that is not much greater than the fan width rating of the spray tip being used.
It is well known that liquid-film atomization can be improved if the liquid is made turbulent or agitated before it passes through the atomization orifice of the airless spray tip. Turbulent or agitated flow of the liquid as it exits the orifice promotes destabilization and disruption of the liquid film, which causes it to break up more readily into finer droplets and into a more uniform spray. For this reason, various types of turbulence promoters have been designed for use with conventional airless spray tips. Such turbulence promoters include various types of pre-orifices, diffusers, turbulence plates, restrictors, flow splitters/combiners, flow impingers, screens, baffles, vanes, and other inserts, devices, and flow networks known to those skilled in the art. Examples of such turbulence promoters and the turbulent flow created in the spray tip are illustrated in the catalog entitled "Airless Nozzles and Accessories", Nordson Corporation, Amherst, Ohio. One such example is a turbulence plate that is inserted into the inlet of the airless spray tip, wherein it divides the flow into two high velocity streams that impinge against one another head on at a ninety-degree angle to the main flow direction. The turbulent discharge then flows through the atomization orifice. Another such example is a restrictor plate that contains a pre-orifice that is somewhat larger in diameter than the atomization orifice. It is so positioned behind the atomization orifice to create a liquid jet that discharges against the atomization orifice, thereby generating the desired turbulence.
The ability to form fine droplets and a feathered spray are principle reasons why air sprays are used instead of airless sprays to apply high quality coatings. The air spray technique accomplishes this by using a large amount of compressed air. However, in contrast, it is well known that the airless spray technique, because it uses no compressed air, deposits the coating composition much more efficiently onto the substrate, that is, it has higher transfer efficiency. Therefore, while it is desirable to utilize airless spray techniques to obtain higher transfer efficiencies, its use is limited to applying low quality coatings because it characteristically does not provide a feathered spray or fine atomization.
Air-assisted airless spray combines features of air spray and airless spray, with intermediate results. It uses both compressed air and high pressure drop across the orifice to atomize the coating composition and to shape the liquid spray, typically under milder conditions than each type of atomization is generated by itself. The air assist helps to atomize the liquid film and to smooth out the spray to give a more uniform fan pattern. Generally, the compressed air pressure and the air flow rate are lower than for air spray. Liquid spray pressures typically range from 200 to 800 psi. However, like an air spray, air-assisted airless spray requires a relatively low viscosity, typically below 100 centipoise, and therefore uses a high concentration of organic solvents. The compressed air usage also typically produces lower transfer efficiency than with airless spray.
As disclosed in the aforementioned patent applications, it has been discovered that supercritical fluids or subcritical compressed fluids are not only effective viscosity reducers, but they can also remedy the defects of the airless spray process by creating vigorous decompressive atomization by a new airless spray atomization mechanism, which can produce the fine droplet size and feathered spray needed to apply high quality coatings.
In the spray application of coatings using supercritical fluids or subcritical compressed fluids such as carbon dioxide, the large concentration of carbon dioxide dissolved in the coating composition produces a liquid spray mixture that has markedly different properties from conventional coating compositions. In particular, the liquid spray mixture is highly compressible, that is, the density changes markedly with changes in pressure, whereas conventional coating compositions are incompressible liquids when they are sprayed.
Without wishing to be bound by theory, it is believed that vigorous decompressive atomization can be produced by the dissolved carbon dioxide suddenly becoming exceedingly supersaturated as the spray mixture leaves the nozzle and experiences a sudden and large drop in pressure. This creates a very large driving force for gasification of the carbon dioxide, which overwhelms the cohesion surface tension, and viscous forces that oppose atomization and normally bind the fluid flow together into a fishtail type of spray.
A different atomization mechanism is evident because atomization occurs right at the spray orifice instead of away from it as is conventional. Atomization is believed to be due not to the break-up of a liquid film from shear with the surrounding air, but instead, to the expansive forces of the compressible spray solution created by the carbon dioxide. Therefore, no liquid film is visible coming out of the nozzle.
Furthermore, because the spray is no longer bound by cohesion and surface tension forces, it leaves the nozzle at a much wider angle than normal airless sprays and produces a "feathered" spray with tapered edges like an air spray. This produces a rounded, parabolic-shaped spray fan instead of the sharp angular fans typical of conventional airless sprays. The spray also typically has a much wider fan width than conventional airless sprays produced by the same spray tip. As used herein, the terms "decompressive atomization" and "decompressive spray" each refer to a spray, spray fan, or spray pattern that has the preceding characteristics.
Laser light scattering measurements and comparative spray tests show that decompressive atomization can produce fine droplets that are in the same size range as air spray systems instead of the coarse droplets produced by normal airless sprays. These fine droplets are ideal for minimizing orange peel and other surface defects commonly associated with spray application. This fine particle size provides ample surface area for the dissolved carbon dioxide to very rapidly diffuse from the droplets within a short distance from the spray nozzle. The coating is therefore essentially free of carbon dioxide before it is deposited onto the substrate.
However, one problem we have noticed when spraying coating formulations with supercritical fluids or subcritical compressed fluids is the high average velocity of the spray. This produces a spray of fine droplets which may have excessively high momentum which causes a portion of the spray to be deflected sideways when the spray impacts upon a substrate. This may reduce transfer efficiency and make electrostatic attraction less effective. Consequently, this detracts from the previously described benefits derived from diluting coating formulations with supercritical fluids or subcritical compressed fluids and spraying the admixture onto a substrate. However, because the decompressive atomization mechanism does not depend upon spraying a coherent liquid out of the spray orifice at high velocity to create high shear with surrounding air, we have discovered that maintaining a sufficiently high velocity is no longer a critical design criterion for the spray tip.
The high velocity or thrust of the spray is believed to be due in part to using conventional airless spray tips that are designed for atomizing incompressible fluids by the liquid-film atomization mechanism, whereas the liquid spray mixture produced by using supercritical fluids or subcritical compressed fluids is highly compressible and produces atomization by an entirely different decompressive atomization mechanism. Thus, to improve liquid-film atomization, conventional airless spray tips are designed to maximize velocity and turbulence as the liquid flows through the atomization orifice for a given pressure drop across said orifice. In particular, in one important design standard, the flow path in the orifice is made very short in order to minimize flow resistance and reduction in turbulence. Furthermore, in addition to being used with turbulence promotion devices, conventional spray tips themselves are sometimes designed to promote turbulent or agitated flow of the liquid at the entrance to the atomization orifice. For example, the spray tip chamber that feeds liquid to the atomization orifice may be contoured or configured such that liquid flows from opposite sides of the chamber converge and impact each other as they flow into the atomization orifice.
The most commonly used airless spray tip design is sometimes called a dome-style spray tip. Such a spray tip is disclosed in, for example, U.S. Pat. No. 4,097,000, issued Jun. 27, 1978. The spray tip of this prior art embodiment is illustrated in FIGS. 1a, 1b, and 1c and is not a part of the present invention. FIG. 1a is a bottom plan view of body 10 and FIGS. 1b and 1c are vertical sectional views, which illustrate fluid flow patterns within feed passageway 15 and through orifice 20, with the aforementioned flow convergence and impaction clearly demonstrated. Body 10 is typically made from a tungsten carbide casting. It contains feed passageway 15 that is a hollow dome-shaped chamber centered about flow axis 30 of the spray tip. This hollow dome is formed in the spray orifice body before final hardening. The hollow dome extends nearly the length of the spray orifice body to such a depth that the roof or wall has the desired thickness at its convergent end. Orifice 20 is sometimes formed by cutting v-shaped groove 25 across the outside end of the dome such that the groove intersects and cuts into the hollow end of the dome. This creates orifice 20 with an elliptical, circular, or similarly shaped cross-section of very short length. The size (cross-sectional area) of orifice 20 is determined by the depth of v-shaped groove 25; that is, a larger orifice passageway produces a larger flow rate.
The angle of v-shaped groove 25 regulates the fan width of the spray, as is known to those skilled in the art. A smaller angle (narrower groove) produces a wider fan. A larger angle (wider groove) produces a narrower fan. The projected face of the discharge end of body 10 can be made in different geometries. A squared projected face is illustrated in FIGS. 1b and 1c. The projected face may also be rounded or domed. Other geometries can also be used, such as a projected face that is squared in the direction perpendicular to the groove but is rounded or domed in the direction parallel to the groove.
Still another dome-style spray tip is disclosed in, for example, U.S. Pat. No. 3,556,411, issued Jan. 19, 1971, which is illustrated in FIGS. 2a and 2b and which represent embodiments which are not in accordance with the present invention.
FIG. 2a shows a spray nozzle assembly that has an externally stepped and internally threaded stainless steel housing 40 that holds a tungsten carbide spray orifice body 50, which is secured in the housing by brazing at 51. The spray nozzle assembly has a turbulence plate 42 that discharges into spray orifice body 50 to promote turbulent flow through the spray orifice. Turbulence plate 42 is held in place by perforate screw 43. The spray nozzle assembly is attached to a spray gun (not shown) at face 41 by the use of a retaining nut (not shown). A gasket is sometimes inserted between face 41 and the spray gun to ensure that a pressure-tight seal is formed. FIG. 2b shows an example of a spray orifice body 50 that can be used in the spray nozzle assembly. It has a circular converging feed passageway 55 that is intersected by groove 65 to form orifice passageway 60.
Other types of dome-style spray tips, which have different mechanical features so as to produce a desired spray pattern, are disclosed in U.S. Pat. No. 3,647,147, issued Mar. 7, 1972; U.S. Pat. No. 3,659,787, issued May 2, 1972; U.S. Pat. No. 3,737,108, issued Jun. 5, 1973; U.S. Pat. No. 3,843,055, issued Oct. 22, 1974; and U.S. Pat. No. 3,754,710, issued Aug. 28, 1973.
A more recent airless spray tip design is called a Cross-Cut.TM. type spray tip, which is disclosed in U.S. Pat. No. 4,346,849, issued Aug. 31, 1982. As illustrated in FIG. 3, which is also not in accordance with the present invention, it is made by cutting interpenetrating grooves, at right angles to each other, into opposite sides of tungsten carbide spray orifice body 70. Groove 75 on the pressurized or inlet side is wedge-shaped in cross-section. Groove 85 on the unpressurized or exit side has a bottom portion that is trapezoidal in crosssection. Orifice 80 is formed by the interpenetration of groove 75 with groove 85. This gives a rectangular-shaped spray orifice passageway that has very short flow path length. The width of outer groove 85 regulates the fan width of the spray.
From the foregoing prior art, it is clear that the design of airless spray nozzles teaches, in part, that it is desirable to maintain a very short orifice path length to maximize turbulent flow from the spray orifice.
Other attempts have been made to obtain a desirable spray pattern from airless spray techniques. U.S. Pat. No. 3,054,563, issued Sep. 18, 1962, discloses a spray nozzle that has an insert to produce swirling motion at high circumferential velocity through the spray orifice, which opens onto a flat face on the top of a ridge and that is flared outwardly so as to form a gore-shaped opening. U.S. Pat. No. 3,858,812, issued Jan. 7, 1975, discloses a spray nozzle for low-pressure sprays, such as agricultural sprays at pressures of 30-60 psi, wherein large droplet size is desired to reduce drifting of sprayed chemicals. The spray nozzle has spaced projections at the entrance end of the nozzle passage leading to the orifice to promote uniformity of the spray. The patent teaches having an inlet chamber or bore that has a larger diameter than the nozzle passage leading to the orifice so that the decrease in diameter causes the liquid to flow to the orifice at a highly accelerated rate, so that the protuberances break up the flow direction through the nozzle passage and thereby produce a turbulent condition.
All of these prior art airless spray nozzle designs are directed to spraying conventional incompressible coating compositions. None of them uses supercritical fluids or subcritical compressed fluids as diluents to spray coating compositions or to spray compressible liquid mixtures.
The aforementioned Smith patents, which are directed to producing fine solid films and powders by using a "molecular" spray, disclose an apparatus which has an apparently elongated heated probe located within the sample collection chamber between the orifice and a transfer line coming from the heating oven. In particular, in the aforementioned Smith U.S. Pat. No. 4,734,227, it is taught that the process utilizes a fluid injection technique which calls for rapidly expanding the supercritical solution through a short orifice into the relatively lower pressure region. The text teaches away from using long orifice designs because, as noted, more conventional nozzles or longer orifice designs would enhance solvent cluster formation, which is taught as being undesirable. Consequently, the function of the apparently elongated probe is to convey, within the sample collection chamber, the fluid between the transfer line, located outside of said chamber, and the prescribed short orifice.
The related aforementioned Sievers, et al. patent similarly teaches the need for rapid expansion to prevent solvent cluster formation of the reagents and similarly utilizes very small orifice diameters of 5 to 50 microns or 1 to 10 microns to spray the dilute supercritical solutions.
Normal airless spray orifices for use with incompressible fluids are designed with the orifice path length as short as possible, so that turbulence induced in the fluid before it enters the nozzle orifice is suppressed as little as possible. It is well known that turbulence causes the coherent liquid film after it issues from said orifice to become unstable sooner and break up into finer drops.
However, as we have discovered, a spray of compressible fluid that contains a supercritical fluid or subcritical compressed fluid, such as carbon dioxide, can atomize at the orifice instead of away from it due to decompressive expansive forces, because the compressibility of the spray mixture causes the supersaturated spray liquid to expand in volume as it undergoes pressure reduction in the orifice. Furthermore, an expanding gas cloud is formed as carbon dioxide gas is released from the spray as it flows from the orifice. This gas cloud significantly reduces the shear that would normally occur with the surrounding air and that would normally significantly reduce the velocity of the spray.
Accordingly, we have discovered that maintaining turbulence and sufficiently high flow velocity are no longer critical design criteria--they are unnecessary due to the vigorous decompressive atomization that can occur when spraying coating compositions with supercritical fluids or subcritical compressed fluids. However, excessively high spray velocity or thrust persists due to the present use of conventional airless spray orifices with this new spray coating technology.
As noted above, the high spray velocity is due in part to the high spray pressure, generally in the range of 1,200 to 2,000 psi, that is used in spraying with supercritical fluids. But lowering spray pressure to resolve the high spray velocity effect is counterproductive, because less supercritical fluid can be dissolved in the coating formulation at lower pressure and the viscosity reduction benefits of the supercritical fluid are not as effectively utilized
One potential solution to the poorer transfer efficiency and electrostatic attraction resulting from the high average spray velocity that we have found is to utilize a very small orifice size, for example a 5 mil orifice size instead of the commonly used 9 mil and larger orifice sizes. This does indeed reduce the spray velocity without being detrimental to good atomization. However, this solution is not constructive, because the smaller orifices suffer from much lower output, and orifice sizes below about 9 mil are also very susceptible to plugging, particularly with pigmented coating compositions.
Clearly, what is needed is an improved means for providing a feathered decompressive spray using an airless spray technique for spraying compressible liquid spray mixtures that contain supercritical fluids or subcritical compressed fluids, wherein the high average spray velocity normally associated with spraying said mixture, which may result in excessively high spray momentum and a portion of the spray being deflected sideways when impacting a substrate, is reduced. This would improve transfer efficiency in general and improve electrostatic attraction to the substrate when electrostatic spraying is used. It can clearly be seen that this problem cannot satisfactorily be solved using conventional airless spray equipment designed for incompressible fluids.